The present invention relates in general to the monitoring of digital multi-program transmissions. More particularly, it relates to monitoring multiplexed packet data streams consisting of digitally encoded video, audio, and data.
Advances in video and audio compression technology, integrated circuit technology, and the communications infrastructure have resulted in a new multiplexed broadcast format for efficient delivery of high quality video and audio programming to consumers, as well as the delivery of useful data services. In particular, the advent of high power communication satellites in combination with the multiplexed broadcast format have allowed over 100 channels of digital video to be transmitted directly to a viewer's home.
For example, digital video information in a multiplexed broadcast format may be transmitted in identifiable groups of bytes or packets. The digital video byte stream is chopped up and packaged into fixed-length packets. The packets from several different video sources can be rapidly assembled together (i.e., multiplexed) onto a single carrier frequency. All packets from a single source form a program or service channel. A single carrier frequency can thus receive and transmit a multiplexed packet stream comprising a number of different video program channels.
In a direct broadcast satellite (DBS) transmission system, for example, a typical high-power satellite transponder is capable of transmitting a single carrier signal containing four live-action television channels or up to eight recorded film channels. A modern satellite can carry dozens of transponders to provide over 100 video channels. Each transponder downlinks its multiplexed or multi-program packet stream to a receiver which resolves the multiplexed packet stream into the original video data channels for display. Similar digital multi-program transmission systems are planned for use in other networks, such as cable networks, computer networks, fiber optic networks and the like.
The broadcast and transmission of packetized data, however, requires new monitoring equipment. For example, ordinary broadcast television can typically be monitored by in-studio personnel simply viewing the live broadcast. With the number of program channels available and high bit rates utilized for digital video transmission, a more sophisticated test system is required. The multiplexed nature of the transmission format does not allow direct access to a particular channel in order to monitor the signal. In-studio monitoring of the broadcast picture of over 100 video channels would also require a large staff of personnel. Furthermore, the forward error correction (FEC) supplied with the transmitted signal corrects and hides signal errors until they reach a level serious enough to cause the received video picture to "freeze" or the complete loss of the transmitted picture. Visual monitoring of the video picture therefore provides little warning of a degrading signal condition before a catastrophic failure of picture quality.
The troubleshooting and isolation of faults in the multiplexed transmission system is also more difficult because of the complex nature of the high-speed digital architecture. The digital transmission system performs the digital conversion, compression, encryption, packetizing, multiplexing and transmission of a large number of signals from various sources at extremely high bit rates. High-speed electronic circuits are required to packetize the digital data and supply the necessary transport control information. At high bit transfer rates, timing errors between circuits may cause packets in the packet stream to be lost. The high data rates and digital nature of the packetized signal, however, makes such errors undetectable to the unassisted human eye. All of the video signals are digitally compressed, encrypted, and packetized so that the video signal cannot be viewed to assist in the troubleshooting of faults. The video picture can only be viewed after the digital signal is depacketized, unencrypted, decompressed, and displayed at the receiver.
The packetized transport architecture of the digital multiplexed transmission system also adds a layer of complexity to the system. The packetized transport architecture requires transport system control information to insure proper de-packetization of the packet stream by the receiver. Errors in the transport system control information may cause the receiver to fail to properly reconstruct the digital data stream.
Accordingly, there is a need for an instrument that can monitor and verify the integrity of the packet stream of digital signals transmitted by a digital multiplexed satellite communication system.